The present disclosure relates to a radioactive-ray imaging apparatus proper for X-ray imaging operations for typically medical cares and nondestructive inspections and relates to a radioactive-ray imaging display system employing such an apparatus as well as transistors employed in the apparatus and the system.
In recent years, a technique making use of a CCD (Charge Coupled Device) image sensor and/or a CMOS (Complementary Metal Oxide Semiconductor) image sensor is the mainstream of imaging techniques based on photoelectric conversion. The imaging technique based on photoelectric conversion is a technique for acquiring an image as an electrical signal. The imaging area of the imaging sensor is limited by the size of a crystal substrate or a silicon wafer. In the field of the medical care carried out by making use of an X ray in particular, however, there are demands for an imaging area having a large size, and the number of demands for a good moving-picture performance is also increasing.
A radioactive-ray imaging apparatus for obtaining an image based on radioactive rays as an electrical signal without making use of a radioactive-ray photographic film is being developed as an imaging apparatus required to have an imaging area with a large size. A typical example of the imaging apparatus required to have an imaging area with a large size is the X-ray imaging apparatus for taking an image of the chest of a human body. Such a radioactive-ray imaging apparatus is an apparatus having a wavelength conversion layer (serving as a fluorescent material) on a circuit substrate including photoelectric conversion devices such as photodiodes and TFTs (thin-film transistors). In the radioactive-ray imaging apparatus, after an incident radioactive ray has been converted into a visible ray, the visible ray is received by the photoelectric conversion device for converting the visible ray into an electrical signal. Then, a circuit employing the TFT reads out the electrical signal, the magnitude of which is determined by the quantity of the visible ray.
In this case, the transistor is created as follows. A plurality of layers are created on a substrate to form a laminated stack having the so-called top-gate structure or the so-called bottom-gate structure. The layers include an electrode layer for the gate, source and drain electrodes of the transistor and the like, a semiconductor layer used for creating the channel of the transistor, a gate insulation film and an interlayer insulation film. However, if a silicon-oxide film is used as the gate insulation film in the transistor having such a structure for example, an X ray may propagate to the inside of the film. If an X ray propagates to the inside of the silicon-oxide film, holes are generated in the film so that a threshold voltage Vth of the transistor is shifted to the negative side as is generally known. (Refer to documents such as Japanese Patent Laid-open No. 2008-252074.)
On the other hand, there has been proposed a transistor allowing the length of the shift of the threshold voltage Vth to be reduced by adoption of a dual-gate structure in which two gate electrodes are provided to sandwich a semiconductor layer. (Refer to Japanese Patent Laid-open No. 2004-265935.)